OPAL - Australia's National Gemstone.
Opal is from the Greek “Opallos” meaning to see a change of colours. It is not only the most beautiful precious gemstone but is also one of the world’s rarest. Opal contains approximately 5-9% of water.
On the Mohs Scale of Hardness opal rates at around 5.5 - 6.5.
Opal is the national gemstone of Australia and 95% of the worlds production of opal comes from Australia. The rest comes from Mexico and Brazil. The first opal was found in 18th century on the border of Hungary and Czech Republic and those specimens are exhibited in the museum in Vienna, Austria.
Opal occurs in very few locations in the world because it requires very special geological and climate conditions.
About 65-140 million years ago during the Cretaceous period, the age of the super continents Pantagea and Gondwana, Central Australia was covered by a giant inland sea. Dinosaurs were the major fauna of the earth at that time; opalised fossil remains of dinosaurs are a legacy of this period.
Approximately 140 million years ago much of Central Australia was an inland sea and fine mineral sands rich in Silica were washed up on its shores. The sea eventually receded and formed what is now known as the Great Artesian Basin. Around 30-40 million years ago, heavy weathering dissolved some of this silica and formed a gel solution. This gel then seeped into crevices and cracks in the ground and over millions of years eventually hardened, forming what we call opal.
SOURCE OF OPAL
Australia produces around 95% of the world's opal. Majority is white or light opal. The rest is black opal and unique boulder opal.
Also found small deposits of opal in Mexico and Brazil.
In late 2008, NASA announced that it had discovered opal deposits on Mars.
TYPES OF OPAL
Natural opal may be transparent through to nearly opaque, although it usually has a “milky” appearance and has a light body tone or white body colour.
Mainly mined in Coober Pedy and Mintabee (South Australia) also found in White Cliffs (NSW).
Black opal is the most valuable of opals and comes from Lightning Ridge, New South Wales.
It is distinguished by the black or very dark body colour of the stone and shows colour within. It can also be a crystal or opaque. It is highly sought after because of its beauty and rarity.
Boulder is an opal that has the host ironstone rock forming naturally as part of the gem. It is a naturally formed solid opal which consists of a fine layer of opal that has been deposited by nature on and in the fissures of ironstone base rock. Because it generally has a black or very dark background colour it has the appearance of black opal.
There are often inclusions in the opal layer and because the layer follows the contours of natural ironstone the shape is often much undulated. Depending on how the stone is polished it can either be very smooth or be "pockmarked" where ironstone protrudes to the surface. The irregular shape of boulder opal makes it a designer’s delight and most pieces of jewelry using boulder opal are very unusual. Mined in Western part of Queensland from north-west Winton down to South-West Quilpie and Yowah.
Opal is considered the birth stone for October or for the star signs of Scorpio and Libra.